1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus that employs an electrophotographic process, such as a copying machine, printer or facsimile machine, and more particularly, to a developing device which uses a two-component developer including a toner and a carrier, and an image forming apparatus having the same.
2. Description of the Related Art
A two-component development system which uses a toner and a carrier as a developer, or a one-component development system which uses only a toner and no carrier are known as methods for a developing device employed in an image forming apparatus based on an electrophotographic method, such as a copying machine, a printer, a facsimile machine, or a composite machine combining these.
In a developing device based on a one-component development system, toner accommodated inside the main body of an apparatus is supplied to the surface of a developing roller by means of a supply roller, and a restricting member is pressed in contact with the developing roller, thereby restricting the amount of toner as well as applying electric charge by friction. Next, by rotating the developing roller and guiding the toner to a development region opposing an image carrying body, an electrostatic latent image which has been formed on the image carrying body is developed. When the toner left on the surface of the developing roller has been returned to the apparatus by passing a sealing member and has been removed from the surface of the developing roller by the supply roller, new toner is supplied to the developing roller. A developing device based on a one-component development system is beneficial in terms of providing a compact developing device at low cost.
A one-component development system of this kind is suitable for improving image quality since there is no disturbance of the electrostatic latent image on the image carrying body by a magnetic brush. However, if a system is adopted in which additional toner can be replenished in order to achieve long life of the developing device, then due to the effects of sorting of the toner particle size and degradation of the toner, or the like, which occurs inside the developing device, a difference in properties arises between newly replenished toner and residual toner inside the developing device. Due to this difference in properties, toner charging defects (charging non-uniformities) become liable to occur when the toner is charged by the restricting member, and there has been a risk that this will lead to decline in image quality due to image fogging, density non-uniformities, and like.
On the other hand, systems proposed as a development system enabling increased life span include a two-component development system which is able to maintain a stable amount of charge for a long period of time and is suitable for achieving long life span of the toner, and a so-called touch-down development system such as that disclosed in Japanese Laid-open Patent Application No. 2003-21961. A touch-down development system employs a two-component development system such as that described above in a toner charging region, with a view to increasing the life of the toner, and in the subsequent development region, employs a one-component development system which propels toner only onto a photosensitive body by a non-contact method, with the object of achieving high image quality. Consequently, such a system is able to utilize the respective benefits of both the one-component development system and the two-component development system. However, with a two-component development system and a touch-down development system, there are limitations on the size reduction and cost reduction of the developing device.
As a method of stabilizing the charge on the toner similarly to a two-component development system, while using a one-component development system, Japanese Laid-open Patent Application No. 2-179662, for example, discloses a developing device which mixes a developer and charge assisting particles which assist charging of the toner in a developer cartridge and supplies previously charged toner only to a developing sleeve via a mesh disposed in an opening section of the developer cartridge.
However, in the method described in Japanese Laid-open Patent Application No. 2-179662, since charge assisting particles are introduced into the developer cartridge, then there are problems in that the cost of consumables increases, and the fact that the ratio between the toner and the charge assisting particles changes and the amount of charge of the toner changes, as the toner is consumed. Furthermore, there is also a problem in that depending on the particle size distribution of the charge assisting particles, blocking of the mesh occurs, and it becomes impossible to supply toner to the development sleeve.